A number of new processes for smelting copper and nickel sulfide concentrates have been adopted on a commercial scale during the past thirty years. Well-known examples of such are the Inco, Mitsubishi, Noranda and Outokumpu processes. Detailed descriptions of these innovations are provided in the patent and technical literature, e.g., Extractive Metallurgy of Copper, Metallurgical Society A.I.M.E., 1976, Vol. 1. Despite the variety of their advantages they all suffer from the important value element content of their furnace slags and the high content of troublesome ultrafine concentrate particulate matter mechanically entrained in their furnace exhaust gases. Furthermore, in addition to copper, nickel, cobalt and the toxic, ubiquitous element, arsenic, valuable, volatile metal and metalloid minor elements are often exhausted in said gases, e.g., antimony, bismuth, cadmium, germanium, indium, lead, mercury, molybdenum, osmium, rhenium, selenium, tellurium, tin and zinc. The furnace matte also contains these impurity elements but a large fraction thereof is conventionally returned to the furnace in converter slag or in converter electrostatic precipitator dust. These elements are present in the furnace slag either in solution as a homogenous mixture or as a heterogenous mixture of disseminated matte entities suspended in the slag matrix. An external slag scavenging procedure, e.g., slag flotation or electric furnace treatment, is frequently employed to decrease loss of values in the furnace slag; and an external dust recovery system, e.g., electrostatic precipitator, bag house, or wet scrubber, is conventionally employed to decrease loss of values in the furnace exhaust gas. Such installations are, furthermore, necessary to prevent escape of toxic elements, e.g., arsenic, cadmium, lead, and mercury, to the environment. It should also be noted that the exhaust gas dust content can be troublesome in the steam boilers usually employed to recover heat from said gas.
It is well known, of course, that conventional copper and nickel reverberatory furnaces suffer seriously from the extravagant cost of their fossil fuel requirements, the undesirably low sulfur dioxide content of the voluminous and dusty furnace gas, the undesirably low value metal concentration of the furnace matte, and the extravagant value metal content of the furnace slag.
The prior art discloses internal furnace slag scavenging procedures for decreasing copper, nickel and cobalt losses in slag by subjecting it to reducing reactions so as to decrease its oxygen potential. Reference is made to the use of iron sulfide, carbon and iron reductants as described by H. H. Stout in U.S. Pat. No. 1,544,048 and by Anton Gronningsaeter in U.S. Pat. No. 2,438,911. However, past attempts to apply concepts of this nature on a commercial scale in the primary furnace have not proven sufficiently rewarding, e.g., the procedure described by one of the present applicants in U.S. Pat. No. 2,668,107.
It is an object of the present invention to improve smelting practice by substantially decreasing the amount of value elements transported out of the furnace by the slag. A further object of the present invention is to improve smelting practice by substantially decreasing the amount of troublesome ultrafine concentrate particulate matter transported out of the furnace by the exhaust gas. An additional object of this invention is to improve smelting practice by decreasing the net cost of effective emission control of particulates, vapors, and sulfur oxides in said gas through maximizing extraction, by vaporization from the concentrate of volatile impurities, thus increasing the concentration of said impurities in the particulates collected and by increasing the concentration of sulfur dioxide in said gas.